1. Field of the Invention
The present invention relates generally to an apparatus and method for receiving a packet data control channel in a mobile communication system. In particular, the present invention relates to a forward packet data control channel receiving apparatus and method for overcoming reception errors in a packet data control channel used for transmitting a control signal for forward packet data in a Code Division Multiple Access (CDMA) system.
2. Description of the Related Art
Mobile communication systems capable of supporting voice and short message services are developing into advanced mobile communication systems capable of supporting a multimedia service such as high-speed packet data and moving image services as well as voice service. For example, a mobile communication system supporting a packet data service includes a CDMA2000 First Evolution-Data Only (1x EV-DO) system supporting only a packet data service and a CDMA2000 First Evolution-Data and Voice (1x EV-DV) system supporting voice and packet data services. Herein, the mobile communication system supporting the packet data service refers to the CDMA2000 1x EV-DV system. The CDMA2000 1x EV-DV system uses a forward packet data channel (F-PDCH) for transmitting packet data to a mobile station, and a forward packet data control channel (F-PDCCH) for transmitting a control signal for the packet data synchronized with the forward packet data channel. A relationship between the F-PDCH and the F-PDCCH will be described herein below with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a configuration of a general CDMA2000 1x EV-DV system. As illustrated in FIG. 1, a base station (BS) 10 is in communication with a plurality of mobile stations (MSs) 21, 22 and 23. To transmit forward high-speed packet data to a particular mobile station, the base station 10 transmits the data over the F-PDCH which is a forward high-speed packet data channel.
The F-PDCCH is a physical channel for carrying a control message that the base station 10 should transmit when there are packets to be transmitted to the in service mobile stations 21, 22 and 23. The F-PDCCH is transmitted the same time as that of F-PDCH for carrying a transmission packet. That is, in order to transmit high-speed packet data, the base station 10 should transmit F-PDCCH together with the F-PDCH. The F-PDCCH has 3 types of slot formats: 1.25 msec (1 slot), 2.5 msec (2 slots), and 5.0 msec (4 slots). The slot formats are selected by a scheduler of the base station 10 every transmission by combining channel information (including a carrier-to-noise ratio (CNR) and a carrier-to-interference ratio (CIR)) and a state of a buffer where transmission data is stored. Herein, the base station 10 does not transmit slot format information (SFI) of the F-PDCCH, determined by the base station 10, to the in service mobile stations 21, 22 and 23. Therefore, the F-PDCCH receivers of the mobile stations 21, 22 and 23 must detect slot format information determined by the base station 10 from a received F-PDCCH signal. Such a slot format detection scheme for a mobile station is called “Blind Slot Format Detection (BSFD).”
FIG. 2 is a diagram illustrating a structure of a forward packet data control channel in a general CDMA2000 1x EV-DV system. In FIG. 2, 1-slot format, 2-slot format and 4-slot format are represented by n=1, n=2 and n=4, respectively. In a 1x EV-DV system, forward packet data control channel information bits (13 bits) transmitted over F-PDCCH refer to a control message. The F-PDCCH uses convolutional codes in order to correct errors occurring in the 13-bit control message, or forward packet data control channel information bits (13 bits), from noises occurring in a transmission channel, and uses Cyclic Redundancy Check (CRC) codes for error detection.
As illustrated in FIG. 2, the control message is input to an adder 31. In addition, because the 1x EV-DV system is a synchronous system, a system time synchronized to a reference time is input to an offset selector 41. The system time is used to randomize information bits transmitted over a forward packet data control channel and convert the randomized information bits into a random sequence. Therefore, a 13-bit random number is received from the system time every 1.25 msec. Accordingly, the offset selector 41 inputs an offset to a Medium Access Control layer Identification (MAC_ID) combiner 32 synchronized with the system time.
The MAC_ID combiner 32 receives 8-bit MAC_IDs for identifying users. In the MAC_ID combiner 32, an 8-bit CRC covered with a MAC_ID is called an “inner frame quality indicator,” and another 8-bit CRC is called an “outer frame quality indicator.” The outer frame quality indicator is exclusive-ORed (or XORed) with an 8-bit binary pattern called a MAC_ID before being transmitted. The reason for XORing a control message with a MAC_ID in the MAC_ID combiner 32 is because double CRCs are used. Therefore, the outer frame quality indicator is represented by an “8-bit CRC-covered MAC_ID.” Here, the MAC_ID refers to a unique number used by a base station in identifying a mobile station.
Information output from the MAC_ID combiner 32 is input to a CRC adder 33. The CRC adder 33 adds an 8-bit CRC to the information output from the MAC_ID combiner 32 so that a receiver can determine whether a received control message is defective. Information output from the CRC adder 33 is input to a tail bit adder 34. The tail bit adder 34 adds 8 tail bits to the CRC-added information. Here, the added tail bits are used for zero state termination of convolutional codes. The CRC structure and detailed blocks thereof will be described in brief. If a 13-bit information word and 8 tail bits are all received, convolutional codes always terminate at a zero state on a trellis in terms of path propagation. Information output from the tail bit adder 34 is input to a convolutional encoder 35. The convolutional encoder 35 performs encoding for correcting an error in a transmission control message from noises occurring in a radio environment of a forward packet data control channel. A coding rate is set differently according to the slot format.
An output of the convolutional encoder 35 undergoes symbol repetition in a symbol repeater 36, and undergoes symbol puncturing in a symbol puncturer 37, and an output of the symbol puncturer 37 is input to a block interleaver 38. The block interleaver 38 block-interleaves input symbols according to the slot format, and the block-interleaved symbols undergo signal mapping in a signal point mapper 39. The mapped symbols after being block-interleaved are multiplied by a channel gain in a channel gainer 40, and then transmitted over a forward packet data control channel.
Aside from a control message for F-PDCH, a receiver can transmit information for correctly recognizing information on a Walsh cover used by a CDMA transmitter. This information is used to transmit Walsh information used by the base station 10 to a mobile station 21, 22 or 23 connected to the base station 10, and is called a “Walsh mask,” and 13-bit information is used for the Walsh mask. If 8 MAC_ID bits are all ‘0’, the base station 10 transmits Walsh mask information used for a 13-bit information word of F-PDCCH. However, if 8 MAC_ID bits are not all ‘0’, the base station 10 transmits a control message (for example, packet size and coding rate) for the F-PDCH transmitted with the 13-bit information word. Therefore, the mobile stations 21, 22 and 23 always check the MAC_ID during the F-PDCCH decoding, and perform different operations according to whether the 8 MAC_ID bits are all ‘0’ as a result of the check.
A structure of a receiver for receiving F-PDCCH in a CDMA2000 1x EV-DV system using F-PDCCH and an example for checking performance of the receiver will be described herein below with reference to accompanying drawings.
FIG. 3 is a diagram illustrating a structure of a F-PDCCH transceiver in a general CDMA2000 1x EV-DV system. Referring to FIG. 3, when data is received, a double CRC adder 51 performs double CRC processing on the received data using the MAC_ID and CRC added thereto, and the double CRC-processed data is coded in a convolutional encoder 52. The coded symbols are subjected to symbol repetition and symbol puncturing in a symbol repeating and puncturing part 53, and then subjected to channel interleaving in a channel interleaver 54. The channel interleaver 54 is used to scatter burst errors occurring in a received signal due to a multipath fading channel. The interleaved symbols are input to a receiver through a channel environment 80.
The receiver is roughly divided into a reception processor 60 and a blind slot format detector 70. The reception processor 60 includes a channel deinterleaver 61, a symbol combining/zero inserting part 62, a Viterbi decoder 63, and a CRC/MAC_ID checker 64. A channel deinterleaver 61 deinterleaves received symbols. A symbol combining and zero insertion part 62 performs a reverse process of the symbol repetition and symbol puncturing process performed for transmission of a forward packet data control channel, on the deinterleaved symbols. A Viterbi decoder 63 decodes convolutional-coded symbols and outputs a control message. A CRC/MAC_ID checker 64 checks CRC and MAC_ID in the control message.
A method for detecting a control message on a forward packet data control channel in the CRC/MAC_ID checker 64 can be roughly divided into the following two methods.
In a first method, a receiver detects an inner CRC from a 13-bit information word and an 8-bit CRC-covered MAC_ID, decoded through Viterbi decoding. The receiver can detect an information word from the CRC check result.
In a second method, a receiver sequentially checks an outer CRC, maintaining the result of the first method. The receiver can detect an information word using both the check result and a result on comparison between the MAC_IDs.
For high-speed data transmission, the CDMA2000 1x EV-DV system employs a Fast Hybrid Automatic Repeat Request (FHARQ) in order to improve the performance of a physical channel. Usually, FHARQ uses N ARQ channels, and the CDMA2000 1x EV-DV system employs N=4 FHARQ. With reference to FIGS. 4A to 4C, an example of N=4 FHARQ will be described herein below. In the drawings, A, B, C and D represent mobile stations that transmit packet data.
As illustrated in FIG. 4A, a base station, or a transmitter, can continuously perform a maximum of 4 HARQ transmissions. For example, whether or not a packet transmitted to a mobile station A is received successfully, the base station can sequentially transmit new packets to a maximum of 3 mobile stations B, C and D for a non-transmission duration until it transmits a next packet to the mobile station A. This is called “N=4 FHARQ,” and this transmission scheme is called “user diversity.” User diversity has been proposed for maximizing the efficiency of channel resources. For example, as illustrated in FIG. 4B, when several mobile stations requesting a packet data service are inactivated, the base station suspends transmission of the F-PDCCH and only noises exist for the non-transmission duration.
For example, as illustrated in FIG. 4C, in N=4 FHARQ, the base station can continuously transmit 4 new packets to the same mobile station A. In this case, the mobile station A continuously receives packets, and all F-PDCCHs received for a no-operation interval (NOI) are targeting the mobile station A. In FIG. 4A, it should be noted that because F-PDCCHs received for transmission durations for the mobile stations B and C, i.e., no-operation interval (NOI) of the mobile station A, are not assigned to the mobile station A, the mobile station A performs no operations. Further, each mobile station should always receive the F-PDCCH assigned thereto and perform a maximally correct operation according to a transmission protocol. In FIG. 4B, because noises received for a no-operation interval (NOI) of the mobile station A, for which no F-PDCCH is transmitted, are meaningless, the mobile station A should not enable its F-PDCCH receiver.
According to the CDMA2000 1x EV-DV standard, a mobile station using F-PDCH for packet transmission demodulates data on the F-PDCH only when the F-PDCCH is assigned thereto. Based on the demodulation result, the mobile station transmits an acknowledgement (ACK) signal or a non-acknowledgement (NAK) signal over a reverse channel. In an actual operation of the system, however, a mobile station may possibly make an error due to noises and disturbances occurring in a channel. The mobile station makes an error in the following cases.
First, an error is made when a mobile station selected by a base station fails to correctly receive the F-PDCCH transmitted by the base station due to noises or disturbances. In this case, because the mobile station fails to recognize whether the F-PDCH is transmitted due to the F-PDCCH error, although it fails to receive a packet or it receives the F-PDCH, the mobile station fails to decode the F-PDCH due to a defective control message and as a result, transmits a NAK over a reverse channel. In this case, however, because the HARQ defined in the 1x EV-DV standard is required, the base station can solve the problem using the HARQ.
Second, an error occurs when a mobile station selected by a base station fails to correctly receive the F-PDCCH transmitted by the base station due to noises or disturbances and, particularly, mistakes the MAC_ID for an all-zero MAC_ID, i.e., Walsh mask update information, due to the F-PDCCH error. In this case, the mobile station changes its own Walsh mask due to the incorrect information. Therefore, although the F-PDCHs are decoded, most of the F-PDCHs suffer from the decoding error because of the Walsh demodulation error. Thus, the mobile station transmits a NAK over a reverse channel, and such an event is continuously repeated unless a correct Walsh mask is generated again. That is, the mobile station always transmits NAKs to the base station. Such an event is illustrated in FIG. 5 by way of example. Referring to FIG. 5, a mobile station performs an incorrect Walsh mask update due to an F-PDCCH false alarm generated at time T1, and continuously generates a F-PDCH error until a time T2.
However, in the second case raising the most serious problem, a mobile station selected by a base station continuously generates an F-PDCH reception error due to the incorrect Walsh mask information unless the base station transmits a new Walsh mask information. Such events can occur in the case of FIGS. 4A and 4B. Therefore, a receiver of the mobile station needs a function capable of diagnosing incorrect Walsh mask information caused by the F-PDCCH error and correcting the incorrect Walsh mask information.
Third, an error is made when a mobile station not selected by a base station mistakes the F-PDCCH transmitted by the base station for its F-PDCCH due to noises or disturbances. In this case, the mobile station decodes F-PDCH, making a mistake that F-PDCH is received. However, the mobile station fails in the decoding and transmits a NAK over a reverse channel.
Fourth, an error is made when a mobile station not selected by a base station mistakes the F-PDCCH transmitted by the base station for its F-PDCCH due to noises or disturbances and, particularly, mistakes the MAC_ID for an all-zero MAC_ID, i.e., Walsh mask update information, due to the F-PDCCH error. In this case, the mobile station changes its own Walsh mask due to the incorrect information. Therefore, although F-PDCHs are decoded, most of the F-PDCHs suffer from a decoding error because of the Walsh demodulation error. Thus, the mobile station transmits a NAK over a reverse channel, and such an event is continuously repeated unless the Walsh mask is updated again. That is, the mobile station always transmits a NAK to the base station.
The third and fourth cases raise no serious problem in forward channels for the following reasons. That is, because the base station knows the MAC_ID of a mobile station that should receive a packet transmitted by the base station, the base station compares the MAC_ID of the mobile station received over a reverse channel, and is allowed to disregard a NAK received from the mobile station and take no action if the received MAC_ID is different from the MAC_ID included therein. However, occupation of a reverse ACK channel (R-ACKCH) for reverse transmission of ACK/NAK and a reverse channel quality indicator channel (R-CQICH) for transmission of the CIR by the non-selected mobile station causes unnecessary occupation of reverse channel resources and interference to the R-ACKCH of a normal mobile station, thereby deteriorating the quality of the R-ACKCH signal from the selected mobile station.
As described above, in CDMA2000 1x EV-DV, a mobile station should accurately analyze the SFI transmitted by a base station, and a reliability factor check should be made on the analysis result on the SFI transmitted by the base station. That is, although the mobile station detects the SFI, it should detect the correct 13 information bits and MAC_ID. If an incorrect information word is received, the mobile station makes the above-stated errors. Above all, an error occurring in the MAC_ID may invite a very serious problem. Such a problem is not fully considered even in the future system. Of course, although the CRC is used as a method for solving such a problem, successive data packets transmitted using the same MAC_ID are all defective due to an error in the MAC_ID for a no-operation interval (NOI), thereby causing deterioration in reliability of the operation of the receiver.